As Tony Lodge has pointed out on this site, the two main parties have swapped plans over HS2. In opposition, the Conservatives wanted to run it north from Heathrow along the M40 corridor – claiming that the new line would thus provide an environmentally-friendly alternative to a third runway at the runway. Meanwhile, in government, Lord Adonis was arguing that the priority was speed, and Labour thus went for the present route, which will run out of Euston. But less than a year later, the two parties had changed clothes. The Coalition Government had plumped for Adonis’s plan, while Labour was supporting a direct link to the airport.

Ministers are often urged on this site not to funk big infrastructure decisions. Some they have: no major decision on airport capacity will be taken until after the next election (talking of Heathrow), a mistake whose origins reach back to David Cameron’s green positioning in opposition. And some they haven’t: whatever one thinks of the Government’s Chinese nuclear deal, George Osborne is striving to keep the lights on, and providing leadership after years of procrastination reaching back through Labour’s wasted years to the Major era. Nor have the Prime Minister and the Chancellor failed to beat the drum for shale gas.

Indeed, the various studies into the plan are a reminder that its outcome is obscure and predictions are uncertain: after all, HS1 is running at only some 30 per cent of capacity. It may be that HS2 helps to open up the north to more business, as investors are whizzed from airports via Euston to Manchester to Leeds in 2034 or so. Or it may be that all the scheme achieves is to help cement London as the core of the country’s growth, as passengers are speeded from northern cities to the pullulating capital. Perhaps, most likely of all, video conferencing and high speed broadband and new technologies will have reduced the demand for the kind of custom for which HS2 is apparently designed.

But even if one believes that HS2 is worth the cost, and wants to trust so many of Britain’s transport eggs to its basket, it surely makes sense first to decide where one’s new airport capacity is going to be, and then run a high speed line north from it – which returns us to the case that the Party pressed in opposition and dropped in government. For the truth is that HS2 is at least as much about politics as economics, as these shifting sands of argument show. Northern voters may be indifferent at best and hostile at worst to the plan. But chambers of commerce and councils in the midlands and north have lined up behind it.

Hence the contortions of Labour’s current position, with Ed Balls seeking to raid HS2’s funds to bolster his sagging credibility, a raging backlash from northern Labour MPs and council leaders…and Ed Miliband nowhere to be seen. But this dimension cuts both ways. To a Tory leadership constantly under attack for being posh and southern, HS2 has become a symbol of its commitment to the north. To pull out of the project would expose Cameron to attack from Labour councils, northern business leaders, an opportunistic opposition front bench…and some of his own MPs. The Prime Minister could be forgiven for thinking that he might as well be filmed attempting to dynamite the Angel of the North.

It may also be that he has come to see HS2 as part of his legacy – like free schools and same sex marriage and the benefits cap. But whether it does or not, the plan will roll on, at least for the time being. Today’s Commons vote is a foregone conclusion. (The Whips, unsurprisingly, are encouraging Conservative dissidents to make an early return to their constituencies.) But a Treasury anxious for money and a Shadow Chancellor desperate for credibility may yet combine to ensure that the HS2 bandwagon never leaves Euston. At any rate, one thing is certain: if the rationale for a plan keeps shifting – and figures and calculations with it – voters should smell a rat.